This study measures the total stress shear strength indices of unsaturated expansive soils at different water contents under drying-wetting cycles by undrained shear tests using the conventional direct shear apparatus. Specifically

the total cohesion and the total internal friction angle are obtained to reflect the undrained shear strength of expansive soils. The matric suctions of soil samples in a fixed shear plane are measured using filter papers after shearing. A model of shear strength for the entire suction range of expansive soils is established based on the results of shear tests. The model is verified by experiment. The drying-wetting cycles significantly reduce the undrained shear strength of expansive soil. The total cohesion weakening is more significant than total internal friction angle reduction. The shear strength index of total stress shows a linear positive correlation with the logarithmic values of matric suction. The shear strength of soil increases nonlinearly with the increase of matric suction

and the increasing rate decreases gradually. The findings confirm that it is feasible to investigate the shear strength of unsaturated expansive soil using the conventional direct shear apparatus and filter paper method.